Treatment of obstructions within passageways is a common challenge faced by medical professionals. These obstructions can occur within body passages such as the ureter, pancreaticobiliary ducts, bowel passages, and airways, or within tubing connected to patients such as external drainage tubing, feeding tubes, intravenous tubes, or chest tubes. Removing these obstructions in a simple and cost effective manner, as well as in a manner involving the least amount of discomfort for the patient as possible, is a goal shared by medical practitioners and medical product manufacturers alike.
In one typical example, calculus or stones in the urinary tract or kidneys arise because of the breakdown of a delicate balance in the body. Specifically, the kidneys must conserve water to function, but they must excrete materials that have a low solubility. These opposing requirements must be balanced during adaptation to diet, climate and activity. The problem is mitigated to some degree because urine contains substances that inhibit crystallization of stone forming minerals. However, when urine becomes supersaturated with insoluble materials, because excretion rates are excessive and/or because water conservation is extreme, crystals form and may grow and aggregate to form a stone.
Although small crystals are readily voided from the kidney with urine, the larger stones may become dislodged from the kidney and enter the ureter or occlude the uretero-pelvic or uretero-vesical junction, causing pain and obstruction. Although some stones can ultimately traverse the ureter, their passage may produce pain and bleeding. The pain can be so severe that narcotic drugs are needed for its control.
Removal of stones from the kidneys or urinary tract can be effected medically or surgically. A well known surgical approach involves passing a flexible basket in a retrograde manner up the ureter from the bladder, and using the basket to capture the stones. Another surgical technique, known as extracorporeal lithotripsy, entails transmission of high-intensity shock waves from outside the body to fragment the stones within the body. The resulting stone fragments are then voided with urine. Yet another surgical technique, percutaneous ultrasonic lithotripsy, requires the passage of a rigid cystoscopy-like instrument in the renal pelvis through a small incision in the flank whereupon stones are broken up by a small ultrasound transducer and removed directly. Another surgical technique is laser lithotripsy via a ureteroscope. All of these procedures, which can be quite painful, are elaborate and expensive, and do not always result in complete removal of the stones and fragments.
Stents are used to decompress ureteral obstruction, ensuring that urine drains from the kidney to the bladder. It has also been recognized that placement of a stent within the ureter can help small stones and stone fragments to transit the ureter. In a typical procedure involving a stent, a guide wire is passed through the ureter to the renal pelvis. A hollow, flexible, cylindrical stent is then advanced with a pusher over the guide wire. The guide wire and pusher are then extracted from the stent and the body, leaving an open lumen for urine to pass through. However, because the lumen defined by the cylindrical stent is even smaller than the ureter itself, all but the smallest stones and sludge are precluded from passing therethrough. Some fragments are able to pass around the ureteral stent but larger stone fragments are unable to pass.